1. Field of the Invention
This invention relates generally to Mach-Zehnder modulators, and more particularly, to a bias-control for optical Mach-Zehnder modulators with voltage-induced optical absorption.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Mach-Zehnder modulators (MZMs) operate by modulating the optical phase difference between two waveguides, which then interfere constructively or destructively to achieve an amplitude modulation (AM) on the output. They have traditionally been made from lithium niobate (LiNbO3) materials.
These LiNbO3 MZMs have been a vital component in modern optical communications systems. These modulators, in various configurations, are able to show good transmitter performance, such as high extinction ratios, low insertion loss, high bandwidth and low transient chirp. These desirable characteristics have led to wide scale deployment of LiNbO3 MZMs in both analog and digital optical communications systems.
However, to maintain good transmitter performance, the phase difference between the two waveguides must be precisely controlled in order to counteract the effects of environmental changes or component aging. Thus, a bias control loop is required to counteract slowly-varying changes in the average phase difference. The bias control loop generates a direct current (DC) compensation signal that keeps the MZM operating about a quadrature point on its transfer characteristics.
DC bias control loop designs for LiNbO3 MZMs are well established in prior art. The control loop typically takes the form of a low-frequency AM dither of the radio frequency (RF) drive signal for the MZM, sensing of that narrow-band frequency component on the output, and adjusting DC bias to keep the low frequency output signal at zero.
With ever increasing demand for capacity, there is a need to reduce cost, power consumption and footprints of all components. Co-packaging laser and modulator into a single package works well in reducing both the footprint and the cost of the transmitter, but further reduction in size and power consumption can only be achieved through monolithic integration of both laser and modulator [1,2]. This potential has led to the development of semiconductor MZMs, such as indium phosphide (InP)-based MZMs.
FIG. 1 is a block diagram of a typical semiconductor MZM 100 that includes an optical input 102, a 1×2 multimode interference (MMI) coupler 104, two modulator arms 106, 108 with either 0° or 180° (PI) phase delays or shifts, respectively, relative to each other, an M0 (0°) electrode 110 on arm 106, an M0-phase (0°) electrode 112 on arm 106, an MP (180°) electrode 114 on arm 108, an MP-phase (180°) electrode 116 on arm 108, and a 2×2 MMI coupler 118, which is the output of the MZM 100. The two outputs 120, 122 of the 2×2 MMI coupler 118 are called DATA 120 and DATABAR_TAP 122. The DATA output 120 is fiber-coupled by a collimating lens 124 to one or more output fibers 126, as well as an optical tap 128, while the DATABAR_TAP output 122 is coupled to a power tap photodiode 130. By applying a voltage on one of the modulator arms 106,108, the phase difference between the two optical waves that propagate through the arms 106, 108 is altered through the electro-optic effect, and this is converted to intensity variations as a result of interference at the output. This results in a theoretical sinusoidal electrical-to-optical (E/O) transfer function in which MZM 100 operates at the quadrature (differential phase of PI/4) point when used as an intensity modulator.
As noted above, to maintain consistent transmitter characteristics over an extended period of time, a MZM control loop is typically required to counter various effects such as drifts, aging of components and temperature variations that prevent the MZM from always operating at the quadrature point. The use of a control loop in MZM is critical and the schemes for controlling a LiNbO3 MZM can broadly be divided into two categories:
(a) Distortion-based LiNbO3 MZM control that seeks to minimize the ratio of even order terms (2nd order typically) to the fundamental, resulting in the MZM always operating at quadrature point of the E/O transfer function [3, 4]. This control scheme uses the fact that at quadrature point, the Taylor's series expansion has non-zero odd order terms with all even order terms identically zero. A typical implementation uses a small amplitude dither signal at frequency Fm applied to the bias voltage. A photodetector is used to provide optical-to-electrical conversion and to detect small variations in optical power as a result of the dither signal. The amplitudes of the fundamental (at frequency Fm) and 2nd order (at frequency 2×Fm) components of the detected signal are measured. The control scheme seeks to change the bias voltage such that the ratio of 2nd order to fundamental of the monitored signal is minimized.
(b) MZM bias-control based on amplitude modulation of a RF drive signal [5]. This control scheme uses the symmetrical property of the sinusoidal E/O transfer function, such that the slopes at any two points equidistant from a quadrature operating point are equal. Thus, an Amplitude Modulation (AM) electrical input signal at quadrature operating point will result in minimum amplitude detected at the AM frequency (Fm Hz), since the out-of-phase AM modulations at the optical-one level and optical-zero level cancel each other given the symmetric nature at the quadrature point. A typical implementation applies a low frequency dither Fm Hz on the gain control of the RF amplifier driver of the MZM. The optical signal is tapped off and detected using a photodetector and, upon optical-to-electrical conversion, the amplitude at Fm Hz is measured. The control scheme seeks to change the operating point such that the detected signal is at a minimum, as any deviation from the quadrature point will result in increase in the amplitude detected, since AM at the optical levels no longer results in perfect cancellations.
The major difference between a LiNbO3-based MZM and that of a semiconductor-based MZM is that, in a semiconductor MZM, the voltage induced phase shifts are accompanied by electro-absorption. This absorption of optical waves is usually non-linear with voltage and results in the heating of the MZM arm.
Thus, the semiconductor MZM differs from the LiNbO3 MZM in two major ways:
(1) The E/O transfer function no longer corresponds to the usual sinusoidal function.
(2) The electro-absorption results in a photocurrent and hence heating of the MZM arm. This causes thermally-induced optical index shifts, which complicates the operation and control of the semiconductor MZM.
The implications of (1) and (2) are that control schemes based on (a) and (b) described above will have difficulty in generating suitable control signals that can be used to bias the semiconductor MZM for proper operation.
In the case of (a), the half-power point of the E/O transfer and the point with maximum slope efficiency no longer coincide with each other, requiring modifications to control schemes that minimize the ratio of 2nd order to fundamental harmonics. Depending on the frequency of the dither signal, the control scheme also has to deal with any additional influence as a result of thermally induced index shifts.
For (b), the thermal index shifts have been shown experimentally to result in a single-sided AM optical output for a reasonable range of extinction ratios. FIG. 2 illustrates the physics of why a single-sided AM optical output is produced, in the presence of optical electro-absorption, using a PI phase shifted semiconductor MZM as an example. In FIG. 2, A is a plot of electrical input, B is a plot of the E/O transfer and electro-absorption versus input voltage, and C is a plot of the optical output with electrical input A. With an amplitude-modulated drive signal shown in A of FIG. 2, in addition to modulating the optical power amplitude, is also modulating the thermally induced index shift as a consequence of absorption, particularly at the voltage that corresponds to a high absorption slope. This thermally induced index shift shifts the transfer curve of the modulator, displacing it in an out-of-phase fashion with respect to the amplitude-modulated drive signal at the high absorption slope region. The out-of-phase transfer curve displacement now tracks the amplitude-modulated drive signal at the low absorption end, canceling or significantly reducing the optical amplitude modulations, thus resulting in a one-sided AM modulated output in which the tapped-off and E/O converted signal will no longer show any minimum signal for the control loop to lock to. This effect is shown in B and C in FIG. 2.
As a result, there is a need for improved control of semiconductor MZMs. Specifically, there is a need for control schemes that prevent bias control loops from erroneously controlling to the wrong operating point due to distortion of the normally sinusoid transfer characteristic. Furthermore, there is a need for control schemes that prevent the heating that is accompanied by optical absorption from causing the control loop to fail to bias the semiconductor MZM at the desired operating point, because the thermal and amplitude modulation effects destructively interfere with each other. The present invention satisfies this need.